Leaf-litter and changing nutrient levels in a seasonally dry tropical hardwood forest,Belize, C.A.

Summary Total above ground plant biomass in a 45 year old seasonally dry tropical hardwood forest was estimated to be approximately 56,000 kg/ha oven dry weight. Nutrients immobilized in the standing vegetation were: N, 203 kg/ha; P, 24 kg/ha; K, 234 kg/ha; Ca, 195 kg/ha; Mg, 47 kg/ha; Na, 9 kg/ha; Mn, 1 kg/ha; Cu, 0.5 kg/ha; Zn, 3 kg/ha; Fe, 4 kg/ha. Total nutrients returned each year through the litter were: N, 156 kg/ha; P, 9 kg/ha; K, 59 kg/ha; Ca, 373 kg/ha; Mg, 32 kg/ha; Na, 5 kg/ha; Mn, 1 kg/ha; Al, 21 kg/ha; Zn, 0.3 kg/ha; Fe, 9 kg/ha. Half of the nutrients immobilized in the standing vegetation were found in the leaves and are returned annually to the soil. Although litter fall is interrupted during the year, the mean nutrient content of the litter was high –5.2%.A decomposition rate of 0.48 percent per day was considered high for a seasonally dry tropical hardwood forest. Fluctuations in soil nutrient levels showed a sharp increase at the start of the rainy season. Later during the dry season nutrient levels decreased to concentrations similar to what they were just prior to the rainy season. Soil organic matter levels were very high –20% in the top 12 cm.     

Biomass and nutrient dynamics in a littoral savanna subjected to annual fires in Congo

The dynamics of nutrient accumulation were studied between two annual fires in a herbaceous savanna of the Congolese littoral region. Trees and shrubs were not studied because of their very low density. After fire the aboveground biomass increased for 10 months up to a maximum of 520 g m^–2. The underground biomass amounted to roughly 630 g m^–2 during the dry season and increased after the fire up to a maximum of 870 g m^–2 during the rainy season. In the aerial parts, the accumulation dynamics differed according to the type of nutrient: Ca accumulation was steady until the following dry season in proportion to the total biomass, while the pattern of K accumulation was similar to the living biomass dynamic and reached a maximum four months after the fire. N, P and Mg followed a middle course. For the underground biomass, N accumulation reached a maximum value at the end of the rainy season (10 g m^–2) and decreased at the beginning of the dry season. Most of this element was incorporated into the root system. During the rainy season, accumulation in the root system was of the same order of magnitude as in the aerial parts for P and Ca, whereas it was much lower for K and Mg. Transfers of nutrients to the atmosphere during the annual burning amounted, respectively, to 85, 25, 39, 21 and 28% of the amounts of N, P, K, Ca and Mg accumulated in the aerial biomass and litter components. Losses during fire were small for P, K, Ca and  Mg compared to the available soil reserves, but not for N. The legume Eriosema erici-rosenii ought to play an important role in N input in this ecosystem.     

Maize productivity and nutrient dynamics in maize-fallow rotations in western Kenya

One-season fallows with legumes such as Crotalaria grahamiana Wight & Arn. and phosphorus (P) fertilization have been suggested to improve crop yields in sub-Saharan Africa. Assessing the sustainability of these measures requires a sound understanding of soil processes, especially transformations of P which is often the main limiting nutrient. We compared plant production, nitrogen (N) and P balances and selected soil properties during 5.5 years in a field experiment with three crop rotations (continuous maize, maize-crotalaria and maize-natural fallow rotation) at two levels of P fertilization (0 and 50 kg P ha^?1 yr^?1, applied as triple superphosphate) on a Kandiudalfic Eutrudox in western Kenya. The maize yield forgone during growth of the crotalaria fallow was compensated by higher post-fallow yields, but the cumulative total maize yield was not significantly different from continuous maize. In all crop rotations, P fertilization doubled total maize yields, increased N removal by maize and remained without effect on amounts of recycled biomass. Crotalaria growth decreased in the course of the experiment due to pest problems. The highest levels of soil organic and microbial C, N and P were found in the maize-crotalaria fallow rotation. The increase in organic P was not accompanied by a change in resin-extractable P, while H₂SO₄-extractable inorganic P was depleted by up to 38 kg P ha^?1 (1% of total P) in the 0–50 cm layer. Microbial P increased substantially when soil was supplied with C and N in a laboratory experiment, confirming field observations that the microbial biomass is limited by C and N rather than P availability. Maize-legume fallow rotations result in a shift towards organic and microbial nutrients and have to be complemented by balanced additions of inorganic fertilizers. Abbreviations: BNF – biological nitrogen fixation; COM – continuous maize; LR – long rainy season; MCF – maize-crotalaria fallow rotation; MNF – maize-natural fallow rotation; SR – short rainy season; TSP – triple superphosphate.     

Nutrient levels in corn and competing weed species in a first year milpa,Indian church,Belize, C.A.

Summary Grain production in two 1/4 hectare plots gave 2971 and 1380 kg/ha for an average yield of 2175 kg/ha. Average yield for local farmers was 1700 kg/ha. Weed biomass accounted for 50% of total bioomass in the hydric Site 4, but held 64–83% of the total nutrients. In the mesic Site 1, values were 5% and 3–29% respectively.Canna edulis was the most abundant weed species. Soil nutrient levels neither increased or decreased significantly between clearing and harvesting. Any decline in future crop production would be due to increased weed competition and not a decline in soil fertility. Weevil (Sitophilus zeamais) infestation is a serious problem in hybrid corn not encountered with local varieties.     

Seasonal Dynamics of Aboveground and Belowground Biomass and Nutrient Accumulation and Remobilization in Giant Reed (Arundo donax L.): A Three-Year Study on Marginal Land

Giant reed (Arundo donax L.) is a perennial rhizomatous grass that shows promise as a bioenergy crop in the Mediterranean environment. The species has spread throughout the world, catalyzed by human activity, though also as a result of its intrinsic robustness, adaptability, and versatility. Giant reed is able to thrive across a wide range of soil types and is tolerant to drought, salinity, and flooding. This tolerance to environmental stresses is significant and could mean that growing energy crops on marginal land is one possible strategy for reducing competition for land with food production and for improving soil quality. We devised an experiment in which we cultivated giant reed in a sandy loam soil with low nutrient availability. Our goal was to evaluate the dynamics of aboveground and belowground biomass and assess the nutrient dynamics of this grass species, focusing particularly on nutrient accumulation and remobilization. The species demonstrated good productivity potential: In the third year, aboveground dry biomass yield reached around 20 t?ha^?1, with a corresponding rhizome dry biomass yield of 16 t?ha^?1. Results for this species were characterized by low nutrient contents in the aboveground biomass at the end of the growing season, and its rhizome proved able to support growth over the spring period and to store nutrients in the autumn. Nevertheless, the adaptability of giant reed to marginal land and the role of its belowground biomass should be investigated over the long-term, and any further research should focus on its potential to reduce greenhouse gas emissions and maintain soil fertility.     

Dry season groundnut stover management practices determine nitrogen cycling efficiency and subsequent maize yields

It is generally thought that grain legume residues make a substantial net N contribution to soil fertility in crop rotation systems. However, most studies focus on effects of residues on crops immediately sown after the legume crop while in fact in many tropical countries with a prolonged dry season there is a large gap before planting the next crop with potential for nutrient losses. Thus the objectives of this study were* to improve the efficiency of groundnut (Arachis hypogaea L.) stover-N (100 kg N ha ^–1) recycling by evaluating the effect of dry season stover management, i.e. surface application and immediate incorporation after the legume crop or storage of residues until next cropping in the rainy season. N dynamics (litterbags, mineral N, microbial biomass N, N ₂O emissions) were monitored and ¹⁵N labelled residues were applied to assess the fate of residue N in the plant–soil (0–100 cm) system during two subsequent maize crops. Recycling groundnut stover improved yield of the subsequent maize (Zea mays L.) crop compared to treatment without stover. A higher N recycling efficiency was observed when residues were incorporated (i.e. 55% total ¹⁵N recovery after second maize crop) than when surface applied (43% recovery) at the beginning of the dry season. This was despite the faster nitrogen release of incorporated residues, which led to more mineral N movement to lower soil layers. It appears that a proportion of groundnut stover N released during the dry season was effectively captured by the natural weed population (54–70 kg N ha ^–1) and subsequently recycled particularly in the incorporation treatment. Despite the presence of weeds major leaching losses occurred during the onset of the rainy season while N ₂O emissions were relatively small. There was a good correlation between soil microbial biomass N and first crop maize yield. Incorporation of groundnut residues led to small increases in economic yield, i.e., 3120 versus 3528 kg ha ^–1 over two cropping cycles in the surface versus incorporation treatments respectively, with corresponding residue ¹⁵N uptakes of 4 and 8%, while ¹⁵N recovery in water stable aggregates (9–15%) was not significantly different. In contrast, when stover was removed and applied before the first crop, yield benefits were highest with cumulative maize yields of 4350 kg ha ^–1 and residue utilization of 12%. However, N recycling efficiency was not higher than in the early incorporation treatment due to an asynchrony of N release and maize N demand during the first crop.     

西双版纳望天树林土壤养分含量及其季节变化

 对西双版纳望天树(Shorea chinensis)林地0～10 cm和10～25㎝土层的有机质,全氮、磷、钾,速效氮、磷、钾等几种营养元素的含量状态及雨季前后的变化进行了研究。结果表明：土壤的养分含量变化为旱季结束＞雨季结束,沟谷地带雨林＞低山地带雨林,并且差异显著;表土0～10 cm是养分富集区,10 cm以下土层的养分含量锐减,两层养分含量对比,除钾元素以外均差异显著,表明望天树热带雨林生存的环境与植被的生活习性,对土壤养分的含量都有一定的影响。雨季对林地土壤的全钾和速效磷含量影响很大,对有机质、全磷和速效钾的含量影响较小,对氮素的影响是低山雨林＞沟谷雨林。雨季期间土壤中的磷素消耗到严重缺乏的水平。     

Decomposition of cowpea and millet amendments to a sandy Alfisol in Niger

Current inputs of organic materials to cropped lands on sandy Alfisols and Entisols in Sahelian West Africa are insufficient to arrest soil organic matter (SOM) decline. Crop residues and green manures require proper management in order to maximize their contribution to nutrient supply and SOM maintenance. The objectives of this study were to quantify the rates of C and N mineralization from cowpea (Vigna unguiculata (L.) Walp.) green manure, cowpea residue, and millet (Pennisetum glaucum (L.) R.Br.) residue under field conditions in Niger and to determine the effect of these organic amendments on pearl millet yield. Millet was grown (1) as sole crop, (2) as intercrop with cowpea, (3) as intercrop with cowpea that was incorporated as green manure during the second half of the growing season, (4) with incorporated cowpea residue (2000 kg ha^–1), (5) with millet residue mulch (3000 kg ha^–1), and (6) with N fertilizer. Carbon loss as CO₂ from soil with and without organic amendment was measured three times per week during the growing season. Nitrogen fertilizer increased millet yield only in a year with a favorable rainfall distribution. Cowpea grown in intercrop with millet during the first part of the growing season and subsequently incorporated as green manure between millet rows increased millet grain yield in a year with sufficient early rainfall, which could be attributed to the rapid rate of decomposition and nutrient release during the first 3 weeks after incorporation. In a year with limited early rainfall, however, densely planted green manure cowpeas competed for water and nutrients with the growing millet crop. Incorporated cowpea residue and millet residue mulch increased millet yield. Surface applied millet residue had high rates of decomposition only during the first 3 days after a rainfall event, with 34% of the millet residue C lost as CO₂ in one rainy season. Recovery of undecomposed millet residue at the end of the rainy season was related to presence or absence of termites, but not to seasonal C loss. Millet residue mulch increased soil organic C content of this sandy Alfisol in Niger. Cowpea and millet residues had a greater effect on SOM and millet yield than cowpea green manure due to their greater rate of application and slower rate of decomposition.     

Influence of gap size and soil properties on microbial biomass in a subtropical humid forest of north-east India

We examined the effects of treefall gap size and soil properties on microbial biomass dynamics in an undisturbed mature-phase humid subtropical broadleaved forest in north-east India. Canopy gaps had low soil moisture and low microbial biomass suggesting that belowground dynamics accompanied changes in light resources after canopy opening. High rainfall in the region causes excessive erosion/leaching of top soil and eventually soil fertility declines in treefall gaps compared to understorey. Soil microbial population was less during periods when temperature and moisture conditions are low, while it peaked during rainy season when the litter decomposition rate is at its peak on the forest floor. Greater demand for nutrients by plants during rainy season (the peak vegetative growth period) limited the availability of nutrients to soil microbes and, therefore, low microbial C, N and P. Weak correlations were also obtained for the relationships between microbial C, N and P and soil physico–chemical properties. Gap size did influence the microbial nutrients and their contribution to soil organic carbon, total Kjeldhal nitrogen and available-P. Contribution of microbial C to soil organic carbon, microbial N to total nitrogen were similar in both treefall gaps and understorey plots, while the contribution of microbial P to soil available-P was lower in gap compared to the understorey. These results indicate that any fluctuation in microbial biomass related nutrient cycling processes in conjunction with the associated microclimate variation may affect the pattern of regeneration of tree seedlings in the gaps and hence be related with their size. This revised version was published online in June 2006 with corrections to the Cover Date.     

Authors index

In a runoff irrigation system in Northern Kenya, we studied the nutrient interactions of sole cropped and alley cropped Sorghum bicolor (L.) Moench and Acacia saligna (Labill.) H.L. Wendl. The trees were pruned once before the cropping season and the biomass was used as fodder for animals. The nutrient contents in leaf tissue, soil and soil solution were monitored and the uptake of applied tracers (¹⁵N, Sr) was followed. The grain yield of alley cropped sorghum was similar to or slightly higher than in monoculture and did not decrease near the tree-crop interface. Foliar N and Ca contents of the crop were higher in the agroforestry combination than in monoculture, corresponding to higher soil N and Ca contents. Soil solution and soil mineral N dynamics indicate an increase of N under the tree row and unused soil N at the topsoil in the alley of the sole cropped trees as well as below 60 cm depth in the crop monoculture. The N use efficiency of the tree+crop combination was higher than the sole cropped trees or crops. Competition was observed for Zn and Mn of both tree and crop whereas for Ca only the tree contents decreased. P, K, Mg and Fe dynamics were not affected by alley cropping at our site. The lower uptake of applied Sr by trees in alley cropping compared to those of the monoculture stand suggested a lower competitiveness of the acacia than sorghum, which did not show lower Sr contents when intercropped. The study showed the usefulness of combining soil and plant analyses together with tracer techniques identifying nutrient competition, nutrient transfer processes and the complementary use of soil nutrients, as the main features of the tree-crop combination. This revised version was published online in June 2006 with corrections to the Cover Date.     

Evaluation of Sesbania speciosa as a green manure for lowland rice in the dry zone,Sri Lanka

Sri Lankan rice farmers rarely practise green manuring and they depend mostly on chemical fertilizers for the nutrient requirement of their crops. With the removal of government subsidy on fertilizers since 1990, they are now faced with the dilemma of meeting the cost of production to sustain previous yield targets. Therefore the present study devotes to evaluate alternative cheaper nutrient sources like green manures for rice. Results of recent work in Sri Lanka have shown that Sesbania speciosa Taub. ex Engl. is a promising green manure for lowland rice in the dry zone. It was evaluated to adopt a suitable green manuring system for rice. Its year-round biomass production, nutrient accumulation during the vegetative growth and nitrogen fixation were monitored. Field trials were conducted to investigate its ability to suppress weed growth when grown during fallow periods and its contribution to soil fertility and effect on rice yield. Highest biomass of around 100 g dry matter per plant, was achieved when S. speciosa was seeded in August and harvested at flowering in November. When this was planted during a fallow period at a spacing of 0.25×0.25 m (16 plants m^-2), percentage weed reduction was as much as 40%. Its N₂ fixation activity was similar to that of S. sesban (L.) Merr., Nitrogen concentration in S. speciosa increased until 70 days after establishment and then decreased towards flowering. Phosphorus and K accumulation was also effective during this period. Therefore suitable time for soil-incorporation of S. speciosa is around 70 days after establishment. S. speciosa-manured and chemically fertilized rice crops were comparable in terms of grain yield, indicating the effective nutrient supplying potential of this green manure. Straw biomass was significantly high under S. speciosa, because of the high N-allocation to vegetative growth of rice under this. This evaluation shows that S. speciosa not only supplies N, but is also an effective and complete supplier of other nutrients for rice. A scheme to introduce this into rice production system is proposed, where S. speciosa is seeded/broadcast two weeks before crop harvest (around January) in Maha (major) season as well as Yala (minor) season (around June) so that its biomass can be incorporated into soil during land preparation of subsequent seasons.     

Woody species and nutrient accumulation during the fallow period of milpa farming in Belize,C.A.

Plant lifeform composition and levels of nutrients accumulated by fallows aged 1, 2 and 3 years under shifting (milpa) cultivation in Belize were measured. Levels of N, P and K allocated to leaves rapidly reached a plateau in 1 year old fallows with little increase in 2 and 3 year old sites. In stem material, K was accumulated rapidly, with little increase after the first year of fallow growth, while N and P accumulation proceeded at steady rates during three years of fallow development. Total biomass in 3 year old fallows averaged 2070 g m^–2 with 10.3 g m^–2 N, 0.73 g m^–2P and 13.2 g m^–2K. Nutrient concentrations in early successional species were higher than in species of later successional status, suggesting different strategies for nutrient utilization.Woody lifeforms dominated the fallow vegetation, accounting for 80% of total biomass in first year fallows and eliminating herbaceous species after 2 and 3 years of fallow growth. The importance of rapid recovery of woody species is discussed as it relates to fallow management and weed control.     

Temporal changes in spatial pattern of fine-root mass and nutrient concentrations in Indian bamboo savanna

Abstract. Temporal variations in the spatial distribution of fine-root mass and nutrient concentrations were studied in recently harvested and mature bamboo savanna sites in the dry tropical Vindhyan region in India. The soil block method and root-free-soil cages were used to investigate fine-root dynamics. The mean annual fine-root biomass was 596 and 690 g/m² in harvested and mature sites, respectively. The fine-root net production calculated by different methods ranged from 486 to 749 g m^-2 yr^-1 in the harvested site and 485 to 875 g m^-2 yr¹ in the mature site. All fine-root mass fractions decreased with increase in distance from the base of bamboo clumps, and the herb root mass showed the reverse trend. Bamboo fine roots were better developed in the 10 - 20 cm soil depth and those of herbs in the upper 10 cm. The ingrowth of fine roots in root-free-soil cages showed maximum biomass accumulation during the rainy season (64.2 - 69.9 g m^-2 mo^-1) and minimum in the summer (4.5 - 7.5 g m^-2 mo^-1). The fine-root nutrient concentrations were strongly related to their diameter. The fine-root nutrient concentrations varied considerably in different seasons. The highest nutrient concentrations in all categories were recorded in summer followed by winter and rainy seasons. Nutrient concentrations in live roots were always greater than those found in dead roots in different diameter classes. We suggest the occurrence of nutrient retranslocation from senescent roots to surviving roots in bamboo savanna. Fine roots in the bamboo savanna increased as a function of N-mineralization and nitrification rates. This tendency further increased after the harvest of bamboo, suggesting the crucial role of fine roots in the bamboo savanna after the harvesting of bamboo culms.     

The effect of flooding on carbon and nutrient standing stocks of helophyte biomass in rewetted fens

Rewetting can strongly affect the matter balance of peatlands. Owing to evidence of increasing CH₄ emissions and P mobilisation after rewetting, the effects of peatland restoration on climate, eutrophication risks and related controversies are discussed. Our study focuses on the role of helophytes in the carbon and nutrient balance of rewetted fen grasslands of NE Germany. We hypothesise that the helophytes Carex riparia, Glyceria maxima, Phalaris arundinacea, Phragmites australis and Typha latifolia differ in biomass production and nutrient standing stock according to site conditions and harvest time. We analysed the helophyte biomass three times a year and continuously measured water levels and quality. Biomass production, nutrient standing stock and litter accumulation were highly species specific and depended on nutrient availability, mean water levels and harvesting time. We conclude that helophytes store considerable amounts of carbon and temporarily improve the water quality by withdrawing high amounts of nutrients from the top soil during the growing season, and by reducing nutrient discharges. Restoring peatlands as effective nutrient and carbon sinks in the landscape should favour highly productive potentially peat-forming helophytes as Phragmites australis by establishing adequate water levels. If nutrients are to be removed from the degraded peatland, then management can combine the restoration of helophyte stands by rewetting with harvesting measures.     

Seasonal variation in the water quality and chlorophyll a of semi-intensive shrimp ponds in a subtropical environment

Twelve water quality variables were measured at different intervals (biweekly and twice a week) in two ponds during two consecutive production cycles in a semi-intensive shrimp farm on the Northwest coast of Mexico. The average harvest during dry season (March–July, a 95 days period) was 1822 kg ha^-1, with an average size of 16.0 g; while the harvest in the rainy season (August–January, a 165-day period) was 2125 kg ha^-1, with an average size of 11.9 g. In the rainy season, dissolved oxygen concentration tended to decrease as the feeding rates and shrimp and phytoplankton biomass were increasing until harvest. During both culture cycles, mean values of temperature, salinity and ammonium in ponds were not significantly different from those in inlet water, whereas pH levels in ponds were higher than in incoming water.Nitrogen nutrients, pH and particulate organic matter levels in ponds and inlet water did not exhibit seasonal differences; the better yield and feed conversion obtained during rainy season reflect the fact that temperature and mean chlorophyll a were higher during the rainy than in the dry season; salinity and total suspended solids were lower during the dry than in the rainy season. The mean levels of the water quality variables in the two ponds were not significantly different in both grow out cycles. Therefore, the differences in productivity between ponds cannot be explained by the water quality.     

Soil nutrients distribution and moisture dynamics on upper catena in a semi-arid nature reserve

The distribution of soil nutrients and soil moisture dynamics on slopes of catenary landscape, were recorded in a semi-arid nature reserve located in the Eastern Transvaal, South Africa. Soil nutrients were accumulating in the slopes of the catena whereas the crests were highly leached. Open grassland bands segregating between the two soil types and their associated plant species, did not show clear stratification of nutrient deposition amongst ground levels down to 1.5 m below the surface. There was however, a tendency of soils with high nutrient contents to correlate with areas of high woody vegetation density. Patterns of soil moisture levels during two annual rainfall periods showed some segregation between soil depths, although the trends indicated that the duration of saturation and dry out of soils levels differed more between rainy seasons than between soil levels during a particular season.     

马尾松人工林土壤有机层和矿质土壤层酶活性随雨旱季的变化

土壤酶活性随雨旱季的变化特征对于深入理解人工林土壤物质循环具有重要意义。2014年6月至2017年4月,采用原位土柱培养方法,研究了长江上游低山丘陵区的马尾松人工林土壤有机层和矿质土壤层转化酶、脲酶和酸性磷酸酶活性随关键时期（雨季初期;雨季中期;雨季末期;旱季初期;旱季末期）的变化特征。结果表明：土壤有机层和矿质土壤层酶活性随关键时期的变化显著,总体表现为雨季高于旱季。土壤有机层转化酶和脲酶活性呈现逐年降低的趋势;矿质土壤层酸性磷酸酶活性呈现逐年增加的趋势。土壤有机层转化酶和酸性磷酸酶活性显著高于矿质土壤层。土壤有机层三种酶活性年际变化幅度及随关键时期的变化幅度均高于矿质土壤层。偏最小二乘分析（PLS）表明,土壤含水量、微生物量和底物含量对土壤酶活性具有显著影响,但其影响大小强烈取决于土壤层次和酶的种类。可见,马尾松人工林土壤有机层酶活性对环境因子变化的响应更敏感,而且驱动土壤有机层和矿质土壤层酶活性动态的主导因子也存在差异。     

Evaluating a sustainability index for nutrients in a short rotation energy cropping system

In dryland environments 3–5 year rotations of tree crops and agriculture represent a major potential bioenergy feedstock and a means to restore landscape hydrologic balances and phytoremediate sites, while maintaining food production. In soils with low natural fertility, the long‐term viability of these systems will be critically affected by site nutrient status and subsequent cycling of nutrients. A nutrient assimilation index (NAI) was developed to allow comparison of species and tree component nutrient assimilation and to optimize nutrient management, by quantifying different strategies to manage site nutrients. Biomass, nutrient export and nutrient use efficiency were assessed for three short rotation tree crop species. Nutrient exports following harvest at 3 years of high density (4000 trees ha^?1) were consistently higher in Pinus radiata, with values of 85 kg ha^?1 of N, 11kg ha^?1 of P, and 62 kg ha^?1 of K, than Eucalyptus globulus and Eucalyptus occidentalis. Component NAI was generally in the order of leaf?1 for N in leaves of P. radiata to 4.7 Mg kg^?1 for P in stem‐wood of E. occidentalis, indicating higher sustainability of wood biomass compared with leaf biomass. The leaves for each species contained between 40 and 60% of the total nutrient contents while comprising around 25–30% of the total biomass. These nutrient exports via biomass removal are similar to those that follow 3 years of wheat production in the same region, indicating there is no additional drawdown of nutrient reserves during the tree cropping phase of the rotation.     

Root length dynamics in agroforestry with Gliricidia sepium as compared to sole cropping in the semi-deciduous rainforest zone of West Africa

Tree root systems may improve soil fertility through carbon inputs, uptake of leachable nutrients and maintenance of soil biomass, but can at the same time reduce crop yields by competition for water and nutrients. Quantitative information about the positive and negative effects of tree roots and their changes in space and time are necessary for the optimization of agroforestry associations. An alley cropping experiment was layed out as a randomized complete block design on a Plinthic Lixisol/Ferralic Cambisol with Gliricidia sepium hedgerows at 5 m distance, including a sole cropping control. The development of root systems was monitored by sequential soil coring (eight samplings) during one year, with maize and groundnut as crops. Additional information is presented from a single sampling for rice during the foregoing year. Pronounced fluctuations of live root length density indicated an important variability in the nutrient and water uptake capacity of the vegetation. At low total root length density, the hedgerows affected the root development in the agroforestry plots directly by the presence of their root systems. At high root length density, they affected root development mainly by improving crop root growth and influencing the composition of the spontaneous vegetation. The root length density of the hedgerows was too low to compete with the crops for soil resources. The hedgerows tended to increase root length densities in the subsoil when few roots were present, thus possibly reducing the risk of nutrient leaching. However, the length density of the perennial root systems decreased during the cropping season, presumably as an effect of repeated pruning, and attained minimum values almost at the same time as the crops. Trees with denser root systems which are less frequently pruned may be more efficient in achieving closer nutrient cycles, though at the cost of higher root competition with crops.     

Soil nutrient dynamics in response to irrigation of a Panamanian tropical moist forest

We measured concentrations of soil nutrients (0–15 and 30–35 cm depths) before and after the dry season in control and dry-season irrigated plots of mature tropical moist forest on Barro Colorado Island (BCI) in central Panama to determine how soil moisture affects availability of plant nutrients. Dry-season irrigation (January through April in 1986, 1987, and 1988) enhanced gravimetric soil water contents to wet-season levels (ca. 400 g kg^–1 but did not cause leaching beyond 0.8 m depth in the soil. Irrigation increased concentrations of exchangeable base cations (Ca²⁺, Mg²⁺, K⁺, Na⁺), but it had little effect on concentrations of inorganic N (NH₄ ^+C, NO₃ ^– and S (SO₄ ^2–). These BCI soils had particularly low concentrations of extractable P especially at the end of the dry season in April, and concentrations increased in response to irrigation and the onset of the rainy season. We also measured the response of soil processes (nitrification and S mineralization) to irrigation and found that they responded positively to increased soil moisture in laboratory incubations, but irrigation had little effect on rates in the field. Other processes (plant uptake, soil organic matter dynamics) must compensate in the field and keep soil nutrient concentrations at relatively low levels.